1. Field of the Invention
The present invention relates to an apparatus for measuring a particle size distribution and more particularly a particle size distribution of a wide range of sample particles is measured by utilizing a diffraction phenomenon or a scattering phenomenon brought about by irradiating dispersed particles with different sources of light.
2. Description of Related Art
In an apparatus for measuring a particle size distribution utilizing a diffraction phenomenon or a scattering phenomenon of a light by particles, an intensity distribution of a diffracted light or scattered light is measured and then the resulting relationship is subjected to a calculation operation based on the theory of Fraunhofer diffraction or Mie scattering to calculate a particle size distribution of sample particles.
FIG. 8 is a perspective view showing a conventional example of an apparatus for measuring a particle size distribution of this type. Referring to FIG. 8, a flow cell 1 is a transparent vessel through which a medium with sample particles dispersed therein flows. The flow cell 1 is irradiated with parallel laser beams L from a laser optical system 2. Separately from the parallel laser beams L, the flow cell 1 is also irradiated with wavelength beams M of a substantial single frequency from a single wavelength optical system 3.
The laser beams L, which have been diffracted or scattered by the sample particles within the flow cell 1, are incident upon a ring-shaped photosensor array 5 through a Fourier transformation lens 4 to determine a particle size distribution of the sample particles. This is effective in determining larger particle diameters from the resulting distribution of the light intensity which is measured.
In addition, single wavelength beams M, which have been diffracted or scattered by the sample particles in the same manner, are incident upon a plurality of photosensors 6a, 6b, etc., which are arranged at spaced positions. These positions correspond to scattering angles different from each other relative to the flow cell 1, to determine a particle size distribution of the sample particles having relatively smaller particle diameters from a distribution of the measured light intensity.
In addition, referring to FIG. 8 again, reference numeral 7 designates a laser diode, reference numeral 8 designates a collimator lens, reference numeral 9 designates a light source, reference numeral 10 designates a spherical mirror, reference numeral 12 designates a collecting lens, reference numeral 13 designates an interference filter, and reference numeral 14 designates a light-measuring slit.
With the above described apparatus for measuring particle size distribution, the laser beams L from the laser optical system 2 and the monochrome beams M from the optical system 3 are incident upon the same flow cell 1 to make the laser beams, which have been diffracted or scattered by the sample particles within the flow cell 1, incident upon the ring-shaped photosensor array 5, and simultaneously make the monochrome beams M, which have been diffracted or scattered by the sample particles, incident upon a plurality of photosensors 6a, 6b. As a result, the particle size distribution of the sample particles having relatively larger particle diameters is measured by the laser beams L, while the particle size distribution of the sample particles having relatively smaller particle diameters is measured by the monochrome beams M. Accordingly, an advantage occurs in that the particle size distribution ranging from smaller particle diameters to larger particle diameters can be measured by means of a single apparatus
However, in the above described conventional apparatus for measuring a particle size distribution, if a concentration of sample particles flowing through the flow cell 1 is changed, then the optical intensity measured by means of the ring-shaped photosensor array 5 and a plurality of photosensors 6a, 6b is also changed, depending upon the change of the concentration of sample particles. That is to say, there is a tendency that the incident light, which has been diffracted or scattered by the sample particles, is more strongly influenced by a multiple scattering with an increase in concentration of sample particles and thus the optical intensity received by the ring-shaped photosensor array 5 and the photosensors 6a, 6b is reduced.
In particular, in the case where the particles have diameters of a submicron order, the multiple scattering can be changed depending upon the wavelength of the irradiated beams, so that an influence of the multiple scattering upon the optical intensity of the laser beams measured by the ring-shaped photosensor array 5 is different from that upon the optical intensity of the monochrome beams measured by a plurality of photosensors 6a, 6b.
However, in the case of the conventional apparatus for measuring a particle size distribution, the influence of the concentration of sample particles upon the measured results has not been taken into consideration, so that problems have occurred in that the measured results are different, depending upon the concentration of sample particles flowing through the flow cell 1 even though the sample particles have the same particle size distribution.
In addition, if a quantity of laser beams emitted from the laser diode 7 and the light source 9, which is a light source of the laser optical system 2 and the single wavelength optical system 3, respectively, fluctuates, then the quantity of light received by the ring-shaped photosensor array 5 and the photosensors 6a, 6b dependently fluctuates so that a problem occurs in that the particle size distribution cannot be accurately determined.